Video Polarity and Direct Injection

[vts1134]

I have a set into which I'd like to directly inject video. My problem is the set only has one video amplifier tube so the video polarity at the grid of the video amp is positive. Injection after the video amp isn't doable as I don't have enough gain, and injection before the video amp isn't doable because the polarity is backwards. Can anyone tell me how flipping the polarity on composite video might be accomplished so that I may inject positive video into the grid of the video amp?